By design, Mode S surveillance radar equipment uses global 24-bit airframe addresses to keep track of the aircraft it is tracking.

Are these addresses also used by higher levels of the ATC system, such as to fill in call signs, flight plans and so forth automatically when the plane first appears in a controller's domain? Or does that still depend on ATC assigning 12-bit squawk codes that pilots set manually?

Does the Mode S address appear in a flight plan when filed?


The Mode S address is not used for identification of aircraft in the higher levels of the ATC system such as the Flight Data Processing System.

Mostly the assigned 12 bit code (also known as Mode A code or squawk code) is used to correlate a radar track to a flight plan. The major exception is that in Europe, when the Mode A code is set to 1000 (octal notation), the Mode-S downlinked Aircraft Identity (ACID) is used for flight plan correlation. This is to free up Mode A codes. Usually planned flights are assigned discrete Mode A codes but using ACID / Flight ID correlation, multiple aircraft on a flight plan in the same area can use Mode A code 1000.

The downlinked ACID must be equal to the Flight ID used to file the flight plan.

The Mode S 24 bit address is only used in low level systems; in the Mode S radar, in multilateration systems, in ADS-B receivers and to some extend in tracking systems.

  • $\begingroup$ The FAA document I found (see other answer) seems to imply that the ACID is also used by Canadian and Australian ATC in similar ways. $\endgroup$ – hmakholm left over Monica Nov 12 '14 at 18:08
  • $\begingroup$ @HenningMakholm That's is correct. As far as I know the ACID is used in Canada and Australia only for correlating ADS-B tracks to flight plans, not for Mode-S radar tracks. I am not 100% sure on that though. $\endgroup$ – DeltaLima Nov 12 '14 at 19:40

(Self-answer after some additional research).

It looks like the 24-bit addresses are not intended for such identification. Instead the Mode S protocol contains a way for the ATC radar to direct a transponder (identified by a 24-bit address) to send its call sign (known as "aircraft identification" in ICAO Annex 10.IV) piggybacked on a normal mode S altitude/squawk reply, coded as up to 8 alphanumeric characters.

It stands to reason that this is what higher-level ATC functions are based on.

Here is one source saying this is the case. It also implies that (some) ATC facilities don't normally assign discrete squawk codes to flights with working Mode S transponders.

(It's not clear to me why the FAA document says the FLT ID is only 7 characters; the data format defined by Annex 10 has room for 8).


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